MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Contact: Diane Ainsworth

FOR IMMEDIATE RELEASE                                           August 26, 1999

RADAR IMAGES CAPTURE BIG, SLOWLY TUMBLING ASTEROID

Astronomers have used the world's two most powerful radar telescopes to 
make the most detailed images ever obtained for an asteroid in a 
near-Earth trajectory.

With an average diameter of about 3.5 kilometers (2 miles), 1999 JM8 is the
largest near-Earth asteroid ever studied in detail. Although this object can 
pass fairly close to Earth in celestial terms, astronomers concur that an actual 
encounter with Earth is not of concern in the next few centuries. 

The new images, obtained with NASA's Goldstone Solar System Radar in
California and the Arecibo Observatory in Puerto Rico, reveal that 1999 JM8
is a several-kilometer-wide object with a peculiar shape and an unusually 
slow and possibly complex spin state, said Dr. Lance Benner, of NASA's Jet 
Propulsion Laboratory, Pasadena, CA, who led the team of astronomers.

"It will take much more data analysis to determine the object's shape and 
exact rotation state," Benner said.  "But just from looking at the images we 
can see that this nearby world is extremely peculiar. At this point we do 
not understand what some of the features in the images are, 
much less how they originated."

The asteroid was discovered on May 13, 1999, at a U.S. Air Force telescope
in New Mexico that is part of the Lincoln Near Earth Asteroid Research
Project, managed by the Lincoln Laboratories of the Massachusetts Institute
of Technology. The discovery provided adequate notice for radar
observations to be scheduled at Goldstone from July 18 to August 8 and at
Arecibo from August 1-9 during the asteroid's close approach to 8.5 million
kilometers (5.3 million miles), the equivalent of 22 Earth-Moon distances. 

"Although Arecibo is the more sensitive telescope, Goldstone is more fully
steerable, and we took advantage of the complementary capabilities of the
two antennas," noted Benner. "The discovery of this object weeks before its
closest approach was a stroke of luck," he said. "The asteroid won't come
this close again for more than a thousand years."

A selection of 1999 JM8 images is available online at
http://echo.jpl.nasa.gov/~lance/1999JM8.html.

Asteroid 1999 JM8 bears a striking resemblance to Toutatis, a
similar-sized, slowly rotating object also studied in detail with radar,
said Dr. Scott Hudson of Washington State University, who is an expert in
using radar images to determine the shapes of asteroids.

"The fact that both these several-kilometer-wide asteroids are in extremely
slow spin states suggests that slow rotators are fairly common among
near-Earth asteroids," he said. "However, although collisions are thought
to be the primary process that determines asteroid spin states, we don't
know how the slow, complex states come about."

The radar imaging technique uses transmissions of sophisticated coded
waveforms and computer determinations of how echoes are distributed in
range and frequency, instead of their angular distribution, as in normal
optical pictures. "Our finest resolution is 15 meters (49 feet) per
pixel, which is finer than that obtained for any other asteroid, even for
spacecraft" said Dr. Jean-Luc Margot, one of the team members from Arecibo
Observatory. "To get that kind of resolution with an optical telescope,
you'd need a mirror several hundred meters across. Radar certainly is the
least expensive way of imaging Earth-approaching objects."

The images show impact craters with diameters as small as 100 meters (330
feet) - about the length of a football field - and a few as large as 1
kilometer (0.6 miles). "The density of craters suggest that the surface is
geologically old, and is not simply a chip off of a parent asteroid," said
Dr. Michael Nolan, a staff scientist at the Arecibo Observatory. "We also
see a concavity that is about half as wide as the asteroid itself, but
we're not sure yet whether or not it's an impact crater."

This is hardly the first time that radar has revealed a near-Earth asteroid
with peculiar characteristics, said Dr. Steven Ostro of JPL, who has led
dozens of asteroid radar experiments.  Radar studies have revealed a
stunning array of exotically shaped worlds with compositions ranging from
solid metal to low-density carbonaceous rock and rotation periods ranging
from 11 minutes to more than a week. "These are very, very strange places,"
he said. "I really envy the coming generations of space explorers who will
visit them."

In addition to Benner, Hudson, Margot, Nolan and Ostro, the radar team
included Drs. Jon D. Giorgini, Raymond F. Jurgens, Donald K. Yeomans and
Martin A. Slade from JPL, and Donald B. Campbell from Cornell University,
Ithaca, NY. 

The radar observations were supported by NASA's Office of Space Science,
Washington, DC. The Goldstone Solar System Radar is part of NASA's Deep
Space Network. The Arecibo Observatory in Puerto Rico is part of the
National Astronomy and Ionosphere Center, which is operated by the Cornell
University under a cooperative agreement with the National Science
Foundation and with support from NASA. JPL is a division of the California
Institute of Technology, Pasadena, CA.

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MEDIA RELATIONS OFFICE
JET PROPULISON LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

IMAGE CAPTION                                        8/26/99

     Radar images of newly discovered 1999 JM8, an unusually 
large asteroid with a slow rotation rate, reveal the object's 
bizarre shape as it streaked past Earth in late July and early 
August at a close approach of 8.5 million kilometers (5.3 million 
miles), about 22 times the distance between the Earth and the 
Moon.  The object, thought to be several miles wide, was captured 
by a radar team led by Dr. Lance Benner of NASA's Jet Propulsion 
Laboratory, Pasadena, CA, after its discovery on May 13, 1999, 
using NASA's Goldstone radar facility in California and the 
Arecibo Observatory's radar in Puerto Rico.  

     The images, clockwise from the top left, were taken on 
August 5, July 28, August 2 and August 1. Radar illumination is 
from the top and the asteroid's rotation is clockwise. The 
Goldstone images taken on July 28 have a vertical resolution of 
38 meters per pixel and those taken on August 1 have a vertical 
resolution of 19 meters per pixel.  The images taken by the 
Arecibo Observatory on August 2 and 5 have a vertical resolution 
of 15 meters per pixel.
 
     1999 JM8 resembles Toutatis, a similarly sized, slowly 
rotating asteroid that also crosses Earth's orbit and that last 
flew past the planet on November 29, 1996, at a close approach of 
5.3 million kilometers (3.3 million miles). Discovery of a second 
large Earth-crosser with a similarly slow spin rate suggests that 
slowly tumbling asteroids are fairly common among near-Earth 
objects. However, although collisions are thought to be the 
primary process determining asteroid spin states, astronomers do 
not know how these slow, complex rotation states come about. 

     1999 JM8 was discovered with a U. S. Air Force telescope in 
New Mexico that is part of MIT's Lincoln Near Earth Asteroid 
Research project. Radar observations by Ostro, Benner and their 
team were supported by NASA's Office of Space Science, 
Washington, DC.  The Goldstone Solar System Radar is part of 
NASA's Deep Space Network.  The Arecibo Observatory, in Puerto 
Rico, is part of the National Astronomy and Ionosphere Center, 
which is operated by the Cornell University under a cooperative 
agreement with the National Science Foundation and with support 
from NASA.  JPL is a division of the California Institute of 
Technology, Pasadena, CA.

                            #####